Abstract: An improved design of the monochromator for pure rotational Raman lidar system is presented. A monochromator system having high spectral resolution and high noise suppression ratio, is designed applying the sixth rank diffraction of the blazed grating. Simulation for the new design indicates that the pure rotational Raman spectral lines of N₂ molecule can be distinguished singly using the sixth rank diffraction of the blazed grating, and the sixth rank diffraction is better than the fifth rank diffraction in controlling the elastic scattering noisy signals.